The present invention relates to a self-piercing rivet for connecting high-strength steels, with a head which has a head diameter and with a shank which has a shank diameter, wherein, at the foot end opposite the head, the shank has an axial recess which has an axial depth, and wherein, at the foot end the shank has a flat surface section.
Furthermore, the present invention relates to a self-piercing riveted joint with at least one upper workpiece and one lower workpiece, of which at least one is formed from a high-strength steel, and with a deformed self-piercing rivet of the type referred to above, the head of which bears against the upper workpiece.
Finally, the present invention relates to a method for producing such a self-piercing riveted joint, with the steps of providing a workpiece arrangement which has at least one upper and one lower workpiece, and of driving a self-piercing rivet of the type referred to above into the workpiece arrangement with a punching force.
A self-piercing rivet of the form referred to above is known from EP 1 229 254 A2. In this document, it is proposed to provide a self-piercing riveted joint with at least two joining parts made of a high-strength steel, which are connected to each other by means of a semi-tubular self-piercing rivet which is formed from steel and which has a rivet head and an adjoining rivet shank with a rivet foot on the end side, wherein the rivet foot is of truncated design in the initial state before the joining operation. The shape of the self-piercing rivet is in this case intended to be identical to the shape of a self-piercing rivet as is known also for joining light metal workpieces, specifically from EP 0 833 063 A1. The truncated design of the rivet foot is intended to achieve a favourable deformation behaviour of the semi-tubular self-piercing rivet during the joining of the high-strength joining parts, wherein the endeavour of the rivet shank to expand is reduced in comparison to a pointed rivet foot. The expansion here is intended to take place only when the rivet shank pierces with the rivet foot into the lower joining part. The joining parts are intended to have a tensile strength of greater than 500 N/mm2 to up to 1500 N/mm2. The tensile strength of the semi-tubular self-piercing rivet used is intended to lie within a range of between 1200 and 1400 N/mm2, but can even reach values of up to 2000 N/mm2.
In order to ensure a suitable expansion behaviour, the quotient from the axial depth of the shank cavity and the outside diameter of the rivet foot is to lie between 0.3 and 0.7. At too small an axial depth of the shank cavity, the rivet shank will not expand sufficiently after perforating the upper steel sheet.
A further semi-tubular self-piercing rivet is known from WO 2007/132194 A1. The shank here is to be provided with a central blind hole bore, wherein the ratio of a difference between an outside diameter and an inside diameter of the shank in the region of the bore to the outside diameter of the shank is intended to lie within the range of 0.47 to 0.52.
Furthermore, EP 2 314 890 A2 discloses a semi-tubular self-piercing rivet for connecting high-strength and super-high-strength steels, wherein a head diameter is generally smaller than or equal to 1.3 times the shank diameter.
However, as before, prior-art self-piercing riveted joints, with which high-strength or super-high-strength steels are connected, may have diverse problems. Firstly, the extent of the expansion may not be symmetrical with respect to a rivet axis. Furthermore, the shank may be compressed and twisted. In some cases, it is not even possible to press the rivet into the workpiece arrangement, with it even being possible for the self-piercing rivet to be fractured.